RLMNotificationToken

@interfaceRLMNotificationToken:NSObject

A token which is returned from methods which subscribe to changes to a Realm.

Change subscriptions in Realm return an RLMNotificationToken instance,
which can be used to unsubscribe from the changes. You must store a strong
reference to the token for as long as you want to continue to receive notifications.
When you wish to stop, call the -invalidate method. Notifications are also stopped if
the token is deallocated.